There have been dramatic improvements in the design and performance characteristics of compact hermetically sealed rechargeable electrochemical cells. These cells are typically configured either as a series of plates or in a spirally wound electrode assembly. The two commonly used chemical systems are the lead acid system and the nickel cadmium system.
Although the lead acid battery system has been known and utilized for many decades, solutions to many of the practical difficulties associated with using such cells were not proposed until the mid-1970s. One of the difficulties seen with early lead acid cells was related to the problem of keeping the electrolyte acid contained within the cell. It was necessary to maintain an excess amount of acid (sulfuric acid) in the cell in order to allow for overcharging of the electrodes during the recharge process. Overcharging leads to the production of hydrogen and oxygen within the cell which traditionally was vented from the cell. Electrochemical cells having vent means and free acid generally had to be held upright in order to prevent the acid from leaking from the cell.
An additional problem with traditional lead acid cells was in maintaining the physical characteristics of the lead plates within the cell. In order to put some "back bone" in the lead plates, lead containing up to one percent of calcium was often used in cells to give the plates some rigidity.
The breakthrough invention in lead acid cells is described in U.S. Pat. No. 3,862,861 of McClelland et al. The McClelland patent discloses the incorporation of several elements that combine to alleviate each of these problems associated with the traditional lead acid cell. The McClelland invention recognized the potential of utilizing the electrochemical recombination reaction. By capitalizing on the "oxygen cycle", a lead acid cell could be produced such that the electrolyte could be maintained in a "starved" condition. Rather than having an excess of electrolyte, the cell could be operated with a minimal amount of electrolyte present in the system. In order to maintain a starved condition, it is necessary to have sufficient absorbent material or pores within the cell to contain the electrolyte while still having space filled with gas.
By using relatively absorptive separator material, McClelland was able to accomplish two distinct functions. The absorptive separator allowed the flow of gases and electrolyte between the positive and negative plates, thereby allowing the oxygen cycle to function. The absorptive separator also acts as a wick to hold the electrolyte within the cell without the necessity of having free electrolyte in the system.
McClelland also discloses a configuration of the plates and separator so that the elements are held tightly together. It was then possible to use considerably purer lead grids that are more corrosion resistant than the calcium-containing lead plates previously used. Venting means are included in the McClelland device as a safety release device to release excess pressure. However, since there is little or no non-absorbed electrolyte in the cell, there is almost no danger of acid leaking from the cell.
Prior to the development of the McClelland device, U.S. Pat. Nos. 3,395,043 and 3,494,800 of Shoeld disclosed the use of relatively thin lead plates in an electrochemical cell. The cells described in the Shoeld patents, being prior in time to the McClelland patent, did not use absorptive, gas permeable separators. The cells disclosed did not, therefore, utilize the oxygen cycle, were not maintained in a starved or semi-starved condition, and probably contained free electrolyte in order to function properly. The Shoeld patents do not indicate that the batteries produced would have superior discharge or recharge characteristics. Based on the techniques and materials available at the time of the Shoeld disclosures, it is quite unlikely that the cell disclosed therein would have had any significant advantages over existing cells.
Since the McClelland patent, there have been several patents disclosing improvements to the fundamental cell disclosed therein. In the field of using blind rivets to assemble an electrochemical cell and to act as an electrical conductor, there is U.S. Pat. No. 3,704,173 by McClelland, et al. U.S. Pat. Nos. 4,465,748 of Harris, 4,414,259 of Uba, 4,233,379 of Gross, 4,137,377 of McClelland and 4,216,280 of Kono each describe separators to be used in starved lead acid cells. U.S. Pat. Nos. 4,725,516 of Okada and 4,648,177 of Uba both identify cell parameters that lead to superior recharge/discharge characteristics in lead acid cells.
U.S. Pat. No. 4,769,299 of Nelson to a certain extent incorporates the inventions of Shoeld and McClelland. The Nelson patent describes the use of grid-like plates and absorptive gas permeable separators as described in McClelland with the extremely thin plates disclosed by Shoeld. The result is a lead acid cell with enhanced recharge/discharge properties.
The theoretical advantage of utilizing thin plates in electrochemical cells has been known for decades. The thinner the plates the less distance electrons have to travel within the plate during discharge, and, during recharge, the shorter distance of non-conductive material to be regenerated. To a certain extent, the thickness of plates utilized has been dictated by the available technology for the production and handling of thin lead films.
U.S. Pat. No. 5,045,415 by Witehira describes a lead-acid battery with extremely thin plates on the order of 5 to 20 micrometers thick (less than 0.001 inches). However, the plates are not interleafed negative and positive plates, but instead are sandwiched together to form thicker plates which, in turn, are interleafed.
U.S. Pat. No. 4,173, 066 by Kinsman is for a laminar battery having a zinc coated cellophane substrate. Of course, the function of a cellophane substrate and the manufacturing concerns associated with it are much different from those of a metal foil substrate. U.S. Pat. No. 3,377,201 by Wagner is for a liquid electrolyte battery such as a lead-acid cell. One embodiment of the invention is a silver-zinc cell having a positive plate 0.010 inches thick but the negative plate is 0.025 inches thick. U.S. Pat. No. 3,023,260 by Coler is for a liquid electrolyte battery having an electrode with a thickness of 0.025 inches. U.S. Pat. No. 4,996,128 by Aldecoa is for a lead-acid battery having a foil thickness of "less than 0.010 inches". The porous paste thickness is not specified, but presumably is greater than the thickness of the foil, to make the total plate thickness in excess of 0.010 inches. Other references to so-called thin plate designs are U.S. Pat. Nos. 4,001,022 by Wheadon (referring to plates in excess of 0.010 thick) and 4,863,728 by Witehira (which describes the use of both thin plates and thick plates in a single battery to provide a variety of discharge characteristics).
The use of thin plates has been seen for some time in alkaline batteries such as nickel-cadmium batteries. For example, U.S. Pat. Nos. 4,963,161 by Chi, 4,937,154 by Moses and 4,539,272 by Goebel, describe alkaline batteries having thin plates. However, alkaline batteries normally are formed with plates of materials with higher tensile strengths than lead which are much easier than lead to manufacture and handle in thin layers.
For much the same reasons that thin plates produce superior results, thin layers of reactive paste also lead to superior discharge/recharge characteristics. The Nelson patent discloses the use of both thin lead grids and thin layers of reactive paste. A basic shortcoming in the Nelson device, is that the paste residing within the grid openings can have a greatly increased distance to the lead plate material. For example, in the Nelson patent the openings in the lead plate grid are constructed so that the distance from the center of the opening to the grid strands is significantly greater than the thickness of the paste layer on the face of the plate. Since the performance characteristics of electrochemical cells is proportional to the thickness of the lead plates and the thickness of the paste layer, the use of grids or other perforated sheets, greatly decreases the efficiency of the cells.
The Nelson patent teaches away from a thin plate design using non-perforated plates, on the grounds that thin plates are prone to corrosion:
"To achieve optimum high rate discharge capability, in theory one would prefer to use thinner plates to reduce the current density on discharge. However, corrosion, particularly at the positive grid as aforementioned, has placed limitations on how thin plates can be made in practice."
Other patents on thin perforated plates include U.S. Pat. Nos. 4,999,263 by Kabata (which refers to films as thin as 3 micrometers coated with a polymeric material having a thickness of "1,000 micrometers or less"; 3,973,991 by Cestaro (referring to perforated lead foil 0.019 inches thick before the application of a coating); and 4,874,681 by Rippel (which refers to a woven perforated plate of strands with a 0.008 inch outside diameter or 0.005 inch outside diameter before application of any coating). Other art in the field includes U.S. Pat. Nos. 4,064,725 by Hug; 4,099,401 by Hug; 4,112,202 by Hug; 4,158,300 by Hug; 4,212,179 by Juergens; 4,295,029 by Uba; 4,606,982 by Nelson; 4,709,472 by Machida; 4,780,379 by Puester; Japanese Patent Nos. 58-119154 and 59-103282 and U.S.S.R. Patent No. 674124.
Of course, the prior art includes many references to thin plate capacitors. See, for example, U.S. Pat. No. 4,720,772 by Yamano. These patents are of marginal relevance, because they are not directed toward battery technology and the plate material is normally aluminum or nickel rather than lead.